fix: avoid implicit bundle target for non-bundle auth resolution

[jmahotiedu]

## Summary

• Stop loading bundle auth configuration implicitly in MustWorkspaceClient unless --target / --environment is explicitly set.

• Keep explicit target behavior unchanged so bundle context remains opt-in.

• Add regression tests covering:

  • Non-bundle command inside bundle directory respects DATABRICKS_HOST.
  • Explicit --target still uses bundle target host.

## Problem
Fixes #4502. Non-bundle commands such as databricks current-user me were resolving authentication from the bundle’s default target when run inside a bundle directory, potentially overriding DATABRICKS_HOST.

## Testing

• go test ./cmd/root

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• PR number: 4504
• Commit SHA: db5cb1821e7e05c06d87783b389b90a59f630f67

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